The present invention relates to a method for producing salt grains having a given particle size.
More specifically, the present invention relates to a method for producing salt grains having a shape and size suitable for the production of cores for moulds used in the manufacture of pressure die-cast and injection moulded articles, to which the following description will make explicit reference without thereby losing any of its general applicability.
The technique of making cores for moulds used in the manufacture of pressure die-cast and injection moulded articles by compacting salt grains inside moulds bearing a negative impression of the desired core has been known for some time. Indeed, the use of cores made in this way drastically reduces both the time taken and the cost of producing these articles by doing away with subsequent machining operations, and also makes it possible to adopt design solutions which could not otherwise be used with the conventional techniques mentioned.
Unfortunately, however, the use of salt cores for the production of pressure die-cast or injection moulded articles is currently limited by the difficulties encountered in obtaining large quantities of salt grains which are suitable for producing these cores at an industrially acceptable cost.
Through experimentation it has been observed that, in order to withstand the high pressures used in the manufacture of the abovementioned pressure die-cast or injection moulded articles, the salt cores need to be extremely compact and, therefore, the shape and particle size of the salt grains used to produce these cores have to be such as to ensure that the maximum possible density is achieved once the salt grains have been compacted inside the appropriate moulds.
At present, salt grains that are suitable for producing salt cores are only obtained after a lengthy and expensive process involving, firstly, a grinding stage in which generic salt agglomerates are fed into grinders from which salt grains of varying particle size are obtained and, subsequently, a grading stage in which the salt grains produced by the grinders are conveyed into screening machines capable of separating from the stream of salt grains those salt grains that are of a suitable size for producing the salt cores referred to above.
The production process described above has the great disadvantage of giving a low specific yield; consequently, in order to ensure that enough salt grains are produced to satisfy the needs of a large plant for producing pressure die-cast or injection moulded articles, the salt grain production plants needed would have to be of a size and would involve costs such as to discourage their use. Furthermore, the salt grains obtained using the production process described above have many sharp edges and their morphological characteristics do not, therefore, particularly lend themselves to being compacted.